Acidity measurements of high acid concentrations in solutions are crucial in many industrial applications, e.g., for optimization of metal processing, for the production of bulk acids, for chemical waste-treatment processes, and for recycling acids. Generally, pH electrodes are not useful at pH's below about 1, and especially at negative pH's such as found in concentrated acid solutions. Continuous samplings and subsequent analyses via titration are often required to monitor ongoing processes involving such concentrated acid solutions. Such samplings and analyses waste both time and materials while generating a separate wastestream.
Fiber-optic sensors including polymer coatings upon the fiber optic waveguide have been previously used to determine high acid concentrations. For example, Carey et al. in Anal. Chem., 1989, Vol. 61, pp. 1674-1678, describe coating thin films, preferably less than about 1 micron, of polybenzimidazole upon lengths of the fiber optic elements, the polymer containing indicators such as Chrome azurol-S and methyl violet B, for determination of nitric acid and hydrochloric acid concentrations. Problems with this system included the need to strip cladding from the initial fiber, refractive index problems such as a tendency for light to be directed out of the fiber into the polymer where the polymer had a higher refractive index than the fiber, the need for long lengths of stripped and coated fiber to obtain sensitivity, and the need for an ultra thin coating to keep light from travelling within the polymer, i.e., when the coating is thinner than the wavelength of the light, the light is passed back into the fiber.
Accordingly, an object of this invention is to provide an improved apparatus capable of determining the acid concentration of high molarity acid solutions.
Another object of the invention is to provide a durable apparatus capable of determining the acid concentration of high molarity acid solutions where such determination is capable of being automatically conducted on-line during an industrial process run.
It is a further object of this invention to provide a method of continuously monitoring the acid concentrations within an on-going process.